Working with Classes and Objects in Java

In this lesson, you will learn

• Defining and declaring a class
• Adding Fields and Methods in a Class
• Creating an Object of a Class
• Accessing Members of a Class
• Examples

Defining a Class

• A class is described as the blueprint or template from which the object is created. It is not a real-world entity itself.
• A class describes a group of objects with the same properties (attributes), behavior (operations), kinds of relationships, and semantics. Person, company, process, and window are all classes. (Michael Balha, James Rumbaugh )
• Anything you wish to represent in the Java program must be encapsulated in a class that defines the state/characteristics and behavior of a basic program known as an object.
• A class creates an object(real-world entity), and an object uses a method to communicate with it.

Example

A dog is a class that defines characteristics such as color, length, height, weight, breed, etc., and behaviors such as jumping, barking, sleeping, walking, and shaking.

These characteristics are known as the member variables, and methods specify the behavior.

Declaring a Class

The use of the class keyword declares a class in Java.

Syntax

class 
{
	//Fields
	datatype instance-var1;
	datatype instance-var2;
	----------------------
	----------------------
	datatype instance-varN;

	// Constructor
	ClassName() {
		// Constructor body
	}

	//Methods
	datatype methodName1(parameter-list)
	{
		//Method Body
	}

	// ..........

	datatype methodNameN(parameter-list)
	{
		//Method Body
	}
}

The data (variables) defined within a class are called instance variables,, and the code is contained within methods.

Collectively, the methods, variables, and constructors defined within a class are called class members.

Constructor: A special method that is called when an instance (object) of the class is created. Constructors have the same name as the class and do not return a value.

Important Note!

Constructors are discussed in the upcoming lessons.

Note: Variables defined within a class are called instance variables because each instance of the class (i.e., each object of the class) contains a separate copy of instance variables.

Example: Adding Fields and Methods inside a Class

class Rectangle
{
	//Fields declaration
	double length;
	double width;

	// Method to display Rectangle Dimensions
	void displayDimensions() 
	{
		System.out.println("Length: "+length);
		System.out.println("Width: " +width);
	}
}

Here, ‘Rectangle’ is a class that defines two instance variables: length and width. We have added a method named displayDimensions() to display a rectangle’s dimensions.

Creating an Object of a Class

An object is an instance of a class and has a unique identity. The following code snippet shows how to declare an object of a class.

type arrayName[arraySize];

Syntax: Declaring an object

Example

Rectangle r1; //r1 is an object name

The code above only declares an object, without allocating memory for it. To allocate memory to the object, you need to instantiate the object by using the ‘new’ operator.

The ‘new’ operator allocates memory to an object. It also returns a reference to that memory location in the object variable.

The following code snippet shows how to instantiate an object.

Instantiate an Object

r1 = new Rectangle(); //r1 is an object name

Now, an object r1 is an instance of the Rectangle class. Thus, it is a “physical” reality and holds space for its member variables.

You can declare and instantiate an object using a single statement as follows.

Rectangle rect1 = new Rectangle(); //rect1 is an object name

There is no limitation on creating objects of a class. You can create multiple objects of a class.

Rectangle rect1 = new Rectangle();
Rectangle rect2 = new Rectangle();
//........
Rectangle rectN = new Rectangle();

When you create an instance (i.e., object) of a class, then each object contains a separate copy of each instance variable defined by the class. Thus, every Rectangle object(rect1, rect2, ……rectN) will contain its separate copies of the instance variable i.e. length, and width as shown in the below figure.

What are the differences between a class and an object?

A class is not an object. (but it’s used to construct them)

A class is a blueprint for an object. It tells the virtual machine how to make an object of that particular type.

Each object of that class can have its own values for that class’s instance variables.

For example, you might use the Button class to create dozens of buttons, each with its own color, size, shape, label, and so on.

Accessing Class Members(Instance Variables and Methods)

The Dot Operator (.)

The dot operator (.) gives you access to an object’s state and behavior (instance variables and methods).

Syntax

objectname.members; //members may be variable or method

Example:

//Accessing and assigning value to a instance variables
rect1.length=10.0;
rect1.width=15.5;

//Accessing method
rect1.displayDimensions();

Example 1: Creating a Rectangle Class

public class RectangleExample1 {
	
	//Fields declaration
    double length;
    double width;
     
    // Method to display Rectangle Dimensions
    void displayDimensions() 
    {
        System.out.println("Length: "+length);
        System.out.println("Width: " +width);
    }

	public static void main(String[] args) {
		RectangleExample1 rect1=new RectangleExample1();
		rect1.length=3.0;
		rect1.width=4.0;
		rect1.displayDimensions();
	}
}

Output:

Length: 3.0
Width: 4.0

Explanation:

This Rectangle class has two fields (length and width), a method displayDimensions() displays the Rectangle’s length and width. The main method creates an object rect1 of class Rectangle and calls the displayDetails() method.

Example-2: Creating a Circle Class

public class CircleExample {
    
    // Fields declaration
    double radius;
    
    // Method to display Circle details
    void displayCircle() {
        System.out.println("Radius: " + radius);
        System.out.println("Area: " + (3.14 * radius * radius));
    }

    public static void main(String[] args) {
        CircleExample circle1 = new CircleExample();
        circle1.radius = 5.0;
        circle1.displayCircle();
    }
}

Output:

Radius: 5.0
Area: 78.5

Explanation:

This program defines a class called CircleExample with one field: radius. In the main method, an object circle1 is created and the radius is assigned a value. The displayCircle() method prints the radius and calculates the area using the formula:

Area=3.14×radius×radius;

This example shows how to:

• Declare a class and field

• Create an object

• Assign values

• Call a method that performs a calculation

Example-3: Creating a Student Class

public class StudentExample {
    
    // Fields declaration
    String name;
    int age;
    
    // Method to display student details
    void displayStudent() {
        System.out.println("Name: " + name);
        System.out.println("Age: " + age);
    }

    public static void main(String[] args) {
        StudentExample student1 = new StudentExample();
        student1.name = "John";
        student1.age = 20;
        student1.displayStudent();
    }
}

Output:

Name: John
Age: 20

Explanation:

This program defines a class StudentExample with two fields: name and age. In the main method, a StudentExample object is created and values are assigned. The displayStudent() method prints the student’s details.

This example demonstrates:

• Storing different types of data (String and int)

• Accessing object properties

• Displaying object information

Example-4: Creating a Car Class

public class CarExample {
    
    // Fields declaration
    String brand;
    int year;
    
    // Method to display car details
    void displayCar() {
        System.out.println("Brand: " + brand);
        System.out.println("Year: " + year);
    }

    public static void main(String[] args) {
        CarExample car1 = new CarExample();
        car1.brand = "Toyota";
        car1.year = 2022;
        car1.displayCar();
    }
}

Output:

Brand: Toyota
Year: 2022

Explanation:

This class CarExample contains two fields: brand and year. An object is created in the main method, and values are assigned to represent a car. The displayCar() method prints the car details.

This example shows:

• How objects represent real-world entities

• Assigning and displaying object data

Example-5: Creating an Employee Class

public class EmployeeExample {
    
    // Fields declaration
    String employeeName;
    double salary;
    
    // Method to display employee details
    void displayEmployee() {
        System.out.println("Employee Name: " + employeeName);
        System.out.println("Salary: " + salary);
    }

    public static void main(String[] args) {
        EmployeeExample emp1 = new EmployeeExample();
        emp1.employeeName = "Alice";
        emp1.salary = 50000;
        emp1.displayEmployee();
    }
}

Output:

Employee Name: Alice
Salary: 50000.0

Explanation:

The EmployeeExample class has two fields: employeeName and salary. In the main method, an employee object is created and values are assigned. The displayEmployee() method prints employee information.

This example demonstrates:

• Using double data type for decimal values

• Representing employee details using a class

Example-6: Creating an Book Class

public class BookExample {
    
    // Fields declaration
    String title;
    String author;
    
    // Method to display book details
    void displayBook() {
        System.out.println("Title: " + title);
        System.out.println("Author: " + author);
    }

    public static void main(String[] args) {
        BookExample book1 = new BookExample();
        book1.title = "Java Programming";
        book1.author = "James Gosling";
        book1.displayBook();
    }
}

Output:

Title: Java Programming
Author: James Gosling

Explanation:

The BookExample class contains two fields: title and author. In the main method, a book object is created and values are assigned. The displayBook() method prints the book details.

This example shows:

• How classes can represent objects like books

• How to store and display textual data.

End of the lesson….enjoy learning.

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